1. Field of the Invention
The present invention relates to the sizing of paper and more particularly it relates to stable dispersions of high solids alkyl ketene dimer sizing agents.
2. Description of the Prior Art
An aqueous dispersion of alkyl ketene dimer forms the basis for a well known and valuable sizing formulation in the manufacture of paper. This value arises mostly from the cost efficiency of the ketene dimer, which bonds directly to the hydroxyl groups on the cellulose fibers. In addition, ketene dimers are noted as sizing agents for paper made at a neutral or slightly alkaline pH.
As an added convenience to the paper maker, ketene dimers are normally sold as aqueous dispersions. These dispersions are generally prepared with cationic starch and sodium lignin sulfonate, because both are inexpensive and have FDA approval.
By emulsion we mean a two phase system with liquid droplets in a continuous liquid medium. By dispersion we mean a two phase system with solid particles in a continuous liquid medium. Alkyl ketene dimer has a melting point of about 45.degree. C. Therefore, mixtures of alkyl ketene dimer in water are emulsions above 50.degree. C. and dispersions below 40.degree. C.
To be useful, the dispersion must remain fluid enough to be pumped and diluted up until the time it is added to the paper machine. Often, this is a matter of weeks, with average temperatures as high as 32.degree. C. If the dispersion could meet these conditions it would be colloidally stable. Historically, it has been difficult to meet these conditions, because of the inherent colloidal instability of these dispersions. An unstable ketene dimer dispersion will increase in viscosity until it cannot be pumped or even until it gels. This problem gets worse as the dimer solids are increased, thus limiting the total solids to less than 15% for most systems.
The process of stabilizing lower solids content alkyl ketene dimer with cationic starch and sodium lignin sulfonate is well known. Savina U.S. Patent 3,223,544 (American Cyanamid) discloses the use of sodium lignin sulfonate or sodium naphthalenesulfonate-formaldehyde resin together with cationic starch to make their emulsion, containing about 6% by weight AKD, agglomerate free, while keeping the pH between 3 and 5.
One example of making ketene dimer dispersions at solids higher than 15% is in Dumas U.S. Patent 4,240,935 (Hercules), disclosing a dispersion of AKD, water soluble cationic resin (e.g. epichlorohydrin derivative), and sodium lignin sulfonate and/or sodium naphthalene sulfonate-formaldehyde resin as anionic surfactants.
There are other examples of high solids AKD dispersions containing synthetic resins. Belgian Patent 817,130 (Tenneco) teaches that the known ketene dimer cationic starch emulsifier combinations having solids content above 12% result in products that increase in viscosity so much that even after one week of storage at ordinary temperatures they turn into pastes that are difficult or impossible to pour, pump or meter. The patent discloses that they have found that stable, high solids ketene dimer dispersions can be prepared without the use of cationic starch using polyvinyl alcohol and sodium ligno-sulphonate as the emulsifiers. Accordingly, they disclose 25% AKD dispersions containing polyvinyl alcohol, sodium lignosulphonate, dicyandiamide/formaldehyde condensate and hydrochloric acid.
German OLS 2,306,542 (BASF) discloses that the common emulsifier for alkyl ketene dimers, i.e. amine modified cationic starches, permit preparation of only dilute dispersions of AKD. They found that higher, i.e. up to 30 weight percent alkyl ketene dimer containing dispersions can be made by adjusting the pH to 3-4.5 and by using a polyvinyl alcohol protective colloid. The pH is adjusted with organic carboxylic acids, for example, acetic acid, propionic acid, lactic acid or particularly with polymeric carboxylic acids, such as polyacrylic acids. Preferably they also add small amounts of benzene, toluene, cyclohexene, or octane to the molten alkyl ketene dimer before emulsification in order to obtain the high solids dispersion at the lower consistency. In addition, the use of surfactants and fungicides is also disclosed. The acetic acid is used in an amount of about 2% based on the dimer even when only a 10% solids dispersion was obtained.
German OLS 2,514,128 (BASF) teaches that it is known to use aqueous dispersions of ketene dimers with amine modified starch; however, these have the disadvantages that they must be used in low solids levels of about 3-8 weight percent in order to avoid dispersions having excessively high viscosity. They disclose that the object of obtaining high solids dispersions having better sizing action was solved by using polyvinyl lactam, such as polyvinylpyrrolidone and polyvinyl caprolactam and anionic, cationic or nonionic emulsifiers. To improve storage stability, they employ water insoluble inert solvents, preferably low boiling hydrocarbons such as benzene, toluene, cyclohexane, etc. The patent teaches adjusting the pH of the emulsions to 2-4.5 with organic acids such as formic, acetic, propionic, or malonic acid advantageously, employing one carboxy group of the water soluble organic acid for each unit of vinyl lactam monomer in the polyvinyl lactams. In addition, the use of fungicides is also disclosed. They disclose obtaining dispersions with 0.01 to 30 percent by weight of ketene dimer.